1. Field of the Invention
This invention generally relates to a mixture of two or more binder materials and the preparation therefrom of green bodies formed from the mixture of such binder materials and a particulate material, such as powdered metals. More particularly, it relates to a binder mixture of components which consolidate at different temperatures so as to enable the formed green body or compact to have one or more of the binder components removed by chemical leaching, rendering the part porous yet structurally stable. The remaining component of the binder system can be removed at a temperature below the final sintering temperature, the part retaining its shape after all binder elements have been removed, thereby leaving only the particulate material so sintering can be completed. Moreover, the foregoing steps are completed under conditions which eliminate or prevent the formation of oxides, thereby substantially reducing the time required for debinding, for removing the remaining thermoplastic portion of the binder under pre-sintering conditions, and due to the absence of oxide formation, for conducting the final sintering cycle. The process also eliminates cracking in the part.
2. Description of the Prior Art
The processes of the prior art for fabricating parts from particulate material have for the most part produced satisfactory products, but they are characterized by slow production rates. Also, they have the disadvantage of being restricted in their choice of, e.g., metal alloys suitable for use in the process because of oxidation of the fabricated part during the course of processing. This oxidation occurs during the debinding step in one widely practiced technique, or it can occur during the feedstock mixing step in another process technique currently being practiced. Further, the known prior processes are slow and/or require expensive special-purpose equipment.
Conventional practice in metal injection molding is that the binders in the feedstock serves two basic functions, that is (1) in liquid form it acts as the carrier in the metal powder/binder slurry which makes it possible to fill the mold cavity uniformly at moderate pressure, and (2) in solid form the binder holds the metal powders together in the desired shape after molding and before sintering. To facilitate sintering, the compact is made porous by removing a portion of the binder prior to sintering. This requires that the binder be made up of at least two components. The first component of the binder remains in the compact to hold the compact together as it is introduced into the sintering furnace, and the second principal component of the binder is stripped from the part to make it porous by thermal or selective leaching means prior to sintering. The first component of the binder which remains in the part must have a high melting or charring temperature in order to bring the metal powders to a pre-sintered condition, as this component of the binder leaves the part in the sintering furnace.
Among the two component feedstock binder systems which are known, e.g., see Adee et al, U.S. Pat. No. 4,225,345, one is a system utilizing a combination of (i) plastics, such as polypropylene, polyethylene, and mixtures thereof, combined with (ii) waxes, such as paraffin, beeswax, carnauba and mixtures thereof. The waxes are the strippable component of the binder, and the plastics are the last component to leave the compact in the sintering furnace. The plastics in general have a melting point ranging above 300.degree. F., and the waxes having melting points ranging from 125.degree. to 200.degree. F.
As a green part is formed in the mold, the part is cooled and the first binder component to solidify, forming a matrix, is the plastic, with the second component to solidify being the wax. In the case of paraffin wax, as it changes phase from liquid to solid, a volumetric shrinkage ranging from 17 to 20 percent occurs. This abrupt shrinkage of the wax component in the already formed plastic matrix of the binder leads to cracking in the green part. This is particularly evident in parts of nonuniform cross-sections where the cooling rate through the part is inherently uneven.
Hermi et al, U.S. Pat. No. 4,283,360, discloses a process for producing molded ceramic or metal parts wherein the binder system is a mixture of resins. One resin is a solvent soluble resin which is removed by dissolution in a solvent, and the other resin is an insoluble resin which is removed by a firing process. Both components of the binder are, therefore, solids. One of the problems of the process, however, is that the debinding process is extremely long in that the dissolution step can be on the order of 50 to about 100 hours. Shorter processing times would be most desirable.
Three additional patents in the prior art of particular interest with regard to binder removal are U.S. Pat. Nos. 4,197,118, and 4,404,166, each to Weitch, and 4,113,480 to Rivers.
The Weitch '118 patent relates to a procedure for the removal of binder to render the green compact porous, and proposes that it be accomplished by thermal evaporation or solvent extraction. However, both of the Weitch techniques for debinding green parts are slow and unduly expensive.
Thermal evaporation, if carried out in a stream of heated air, produces a brittle, heavily oxidized compact. This is the most common state-of-the-art method presently employed. For example, when processing nickel/iron alloys, debinding in air takes greater than two (2) days. A great volume of pre-heated air is also required in the thermal evaporation process, which is a costly and inefficient use of energy. The heavily oxidized compacts produced by thermal evaporation also require a long oxide reduction step in a reducing atmosphere before the sintering cycle can be initiated in the furnace, requiring a total cycle time of 2 to 3 days. Further, metals or alloys which form stable oxides cannot be processed by this technique.
The solvent extraction technique disclosed by Weitch requires that the green compact be pre-heated in the absence of solvent prior to introducing the solvent in vapor form. It is well known in industrial practice that vapor degreasing is most effective when cold parts are introduced into the vapor of a solvent because the clean solvent then condenses on the surface of the cold part, and cleaning action is vigorous. The condensing rate, and subsequent cleaning action, is diminished as the part heats up to the temperature of the condensing solvent. Weitch has applied this well-known principal to avoid damage to green compacts by pre-heating the compact, thus slowing the cleaning rate. The result is an extended debinding time and a process requiring special equipment. The quantity of solvent and time required for solvent extraction as described in the Weitch patent is impractical. This is evident from the present predominant use of thermal evaporation by the industry.
The Rivers' process utilizes a fluid mixture of water, methyl cellulose, glycerine and boric acid, as the carrier mixed with metal powders at room temperature to form a moldable slurry. The green compact is formed in a mold by elevating the temperature of the slurry in the mold which causes the methyl cellulose to reject water and form a gel. It has been found that active powders, such as reduced iron, will react with the oxygen in the water to produce iron oxide and generate an exothermic reaction. This causes the mixture to heat and set up prior to molding and limits this process to less active, or pre-alloyed metal powders.
Accordingly, it is an object of the present invention to provide a novel process for fabricating parts from particulate material which overcomes the aforedescribed deficiencies of the prior art, and especially eliminates cracking in the part.
It is another object of the present invention to provide such a process which also eliminates or prevents the formation of oxides.
Still another object of the present invention is to provide such a process which substantially reduces the time required for debinding.
These and other objects, as well as the scope, nature and utilization of the invention, will be apparent to those skilled in the art from the following description and the appended claims.